loader.s

global loader                 ; the entry symbol for ELF
extern kmain

ALI      equ 1 << 0           ; align loaded modules on page boundaries
MEMINFO  equ 1 << 1           ; provide memory map
FLAGS    equ ALI | MEMINFO    ; the multiboot flags field
MAGIC    equ 0x1BADB002       ; define the magic number constant
CHECKSUM equ -(MAGIC + FLAGS) ; calculate the checksum (magic number + flags + checksum should equal 0)
KERNEL_STACK_SIZE equ 4096    ; size of stack in bytes

section .text:                ; start of the text (code) section
align 4                       ; the code must be 4 byte aligned
    dd MAGIC                  ; write the magic number to the machine code
    dd FLAGS
    dd CHECKSUM               ; and the checksum
loader:                       ; the loader label (defined as entry point in linker script)
    mov esp, kernel_stack + KERNEL_STACK_SIZE ; point esp to the start of the
                                              ; the stack (end of memory area)
    call kmain
.loop:
    jmp .loop                 ; loop forever

section .bss
align 4                       ; align at 4 bytes
kernel_stack:                 ; label points to beginning of memory
  resb KERNEL_STACK_SIZE      ; reserve stack for kernel